Means and method for reducing carbon dioxide to provide a product

ABSTRACT

A process for reducing carbon dioxide to at least one useful product includes two redox couple electrolyte solutions separated by a first membrane having photosensitizers. The carbon dioxide to be reduced is separated from one of the redox couple electrolyte solutions by another membrane having a catalyst. Water provides hydrogen ions which participate in the reduction of carbon dioxide via a separator. In the operation both membranes are illuminated and produce excited solar sensitizers which cause electron transfer from a first redox solution to the second redox solution and then to the carbon dioxide to react with the hydrogen ions, reducing the carbon dioxide to provide at least one product.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photoelectrical chemical methods and apparatus for reducing carbon dioxide.

2. Summary of the Invention

A process and apparatus for reducing carbon dioxide to at least one useful product includes two redox couple electrolyte solutions separated by a first membrane having photosensitizers. The carbon dioxide to be reduced is separated from one of the redox couple electrolyte solutions by another membrane having photosensitizers and a catalyst. Water provides hydrogen ions which participate in the reduction of the carbon dioxide via a separator. in operation both membranes are illuminated and produce excited photosensitizers which cause electron transfer from a first redox solution to a second redox solution and thence to the carbon dioxide thereby, in cooperation with the hydrogen ions, reducing the carbon dioxide to provide at least one product.

The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a photo system constructed in accordance with the present invention for reducing carbon dioxide to at least one useful product.

FIG. 2 is a schematic energy diagram of the electron transfer of the system shown in FIG. 1.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a photochemical reduction cell 1 made of material which permits the passage of light while not passing a fluid, is divided into three chambers 5, 6 and 7 by membranes 10 and 11. Membranes 10 and 11, which may be made of Nafion, contain photosensitizer material represented by blocks 12 and 17, respectively. Further membrane 11 also contains a catalyst (not shown) on the surface that will be in contact with the CO₂, as hereinafter explained. Membrane 10 with photosensitizers 12 may be also referred to as photosystem II, while membrane 11 with photosensitizers 17 may also be referred to as photosystem I.

Photosystem II may use photosensitizers 12 from the following elements: n-TiO₂, n-Fe₂ O₃, n-WSe₂, p-InP, methylene blue or porphyrins while photosystem I may use photosensitizers 17 from the following elements: p-InP, p-WSe₂, p-GaAs, p-CdTe, n-WSe₂, n-CdSe, Ru(bipy)₃ Cl₂ or phthalocyanines.

Chamber 5 contains an aqueous electrolyte , while chamber 6 contains another aqueous electrolyte. By way of example, the solution in chamber 5 may have a redox system R_(II) /O_(II) from the following redox systems: H₂ O/O₂, Br⁻ /Br₂, H₂ /H₂ O, methylene blue, Fe⁺² /Fe⁺³ or EDTA. Chamber 6 may have a redox system R_(I) /O_(I) from the following redox systems: I⁻ /I₂, S²⁻ /S_(n) ²⁻, triethanolamine or methyl viologen.

The following table I shows preferred combinations of R_(II) /O_(II), R_(I) /O_(I), photosystem II and photosystem I.

                  TABLE I                                                          ______________________________________                                         R.sub.II /O.sub.II                                                                     Photosystem II                                                                             R.sub.I /O.sub.I                                                                         Photosystem I                                    ______________________________________                                         H.sub.2 O/O.sub.2                                                                      n-TiO.sub.2, n-Fe.sub.2 O.sub.3                                                            None      p-InP or p-WSe.sub.2,                                                          p-GaAs or p-CdTe                                 Br.sup.- /Br.sub.2                                                                     n-WSe.sub.2 I.sup.- /I.sub.2                                                                         n-WSe.sub.2                                      H.sub.2 /H.sub.2 O or                                                                  p-InP       S.sup.2- /S.sub.n.sup.2-                                                                 n-CdSe                                           methylene                                                                      blue                                                                           Fe.sup.+2 /Fe.sup.+3                                                                   methylene blue                                                                             Triethanol-                                                                              Ru(bipy).sub.3 Cl.sub.2                                              amine                                                      EDTA    Porphyrins  Methyl viologen phthalocyanines                            ______________________________________                                    

The nature of the CO₂ predominant reduction product is highly dependent upon the catalyst on the surface of membrane 11. For example, if the catalyst is a nickel amalagam, zinc amalagam or lead, the predominant product is formic acid. Other typical products may be formaldehyde, methanol and methane. Materials for this purpose may be selected from Table II.

TABLE II

Chalcogenides (Ni, Co, Mo, W): formed in situ

Titanium oxide: thin-filming coating or particles

SrTiO₃ : thin-film coating or particles

NADPH (reduced nicotinamide adenine dinucleotide phosphate)

Metal coatings: Pb, In

Amalgams (Ni, Zn)

Chrysene

Ethyl acrylate

Phthalocyanines (Ni, Co)

Porphyrins

Methyl viologen: dissolved species

A source 30 provides CO₂ through a valve 32 to chamber 7 in reduction cell 1. Another chamber 35 is formed by a separator 36 which also permits the passage of light and hydrogen ions but not electrolyte. A source 40 provides water through a valve 42 to chamber 35. The reduced CO₂ or product is drawn off by way of a line 44 via pump 47 through a valve 50 and provided to storage means 55.

A biasing circuit may be used to improve the electron transfer. Such a circuit includes an electrode 60 which is connected to the negative terminal of a battery 64 whose positive terminal is connected to a potentiometer 68. Potentiometer 68 is connected to another electrode 70.

THEORETICAL DISCUSSION

The left surface of membrane 10 subjected to direct illumination will typically consist of an n-type semiconductor material thereby generating a negative photopotential. This negative photopotential is a result of excitation of electrons from the valence to conduction band of the semiconductor material. Electron holes left in the valence band will drive the oxidation of R_(II) →O_(II) on the illuminated side of PS II. R_(II) /O_(II) represents reduced/oxidized species in the solution. Ideally, as in the case of natural photosynthesis, this couple should be H₂ O/O₂. In practice, however, not many n-type semiconductors are stable enough to evolve oxygen. Those that can do it (TiO₂, SrTiO₃, Fe₂ O₃) have relatively large band gaps, which means they do not absorb a major portion of sunlight. When a low band gap material such as n-WSe₂ is used, a redox couple such as Br⁺ /Br₂ has been found necessary to achieve semiconductor stability. Operation with H₂ O/O₂ would lead to photocorrosion effects at the semiconductor. With the use of battery 64, potentiometer 68 and auxiliary electrode 60, the oxidized species O_(II) (such as bromine) is reduced back to R_(II) (such as bromide). The auxiliary electrode 70 is biased with a positive voltage so that it oxidizes water species to oxygen.

While an oxidation reaction occurs on the left side of PS II, a corresponding reduction reaction O_(I) →R_(I) occurs on the right side. This occurs because electrons in PS II become photoexcited to higher energy levels (i.e., more negative potentials), as shown in FIG. 2. The left side of PS I will also interact with R_(I) /O_(I). Thus, R_(I) /O_(I) acts like a charge transfer relay. The equilibrium electrochemical potential of R_(I) /O_(I) is more negative than R_(II) /O_(II) ; consequently, when excitation of the semiconductor material corresponding to PS I occurs, its photoexcited electrons will be able to reach much higher levels compared to the photoexcitation level of PS II. Electron holes will oxidize R_(I) to O_(I) on the left side of PS I. On its right side, the highly energized photoexcited electrons will have the potential to reduce CO₂ species. However, competing reactions such as reduction of water species to hydrogen will also occur. It is here that the role of surface catalyst, previously referred to in Table II, will be important. The catalyst for each semiconductor/electrolyte system will be chosen to efficiently produce the most desirable reaction product (e.g. Pb, In, or Hg to produce formic acid).

For both PS I and PS II, n- or p-type semiconductor materials can be used depending upon the direction of illumination. n-type semiconductors will generate a negative photopotential, drive an oxidation reaction on its illuminated surface, and a reduction reaction on its dark side. p-Type material will generate a positive photopotential, drive a reduction reaction on its illuminated surface, and an oxidation reaction on its unilluminated side. The net effect of the overall process will be an electron movement through the photo-reducing membrane from the left side to the right side. To keep electroneutrality in the system, this charge movement has to be balanced by equal migration of positive charges. Nafion membranes are good cation exchangers for species such as H⁺. The protons will be used on the right side with CO₂ to produce, for example, formic acid. Thus water is consumed in the overall process for CO₂ reduction.

Separator 36 on the right side of the cell is provided to pass the hydrogen ions while preventing intermixing of the reduced CO₂ species with oxygen produced at the positive auxiliary electrode. The oxygen evolved will be vented out from the system. Battery 64, potentiometer 68 and the electrodes 60 and 70 may be used to increase the current necessary for the CO₂ reduction process.

The present invention as hereinbefore described is an electrophotochemical method and apparatus for reducing CO₂ to a useful product. 

What is claimed is:
 1. A process for reducing carbon dioxide to at least one useful product comprising the steps of:a. providing a R_(II) /O_(II) redox coupled electrolyte solution, b. providing a R_(I) /O_(I) redox coupled electrolyte solution, c. separating the R_(II) /O_(II) solution from the R_(I) /O_(I) solution with a first membrane having photosensitizers, d. providing carbon dioxide, e. separating the carbon dioxide from the R_(I) /O_(I) solution with a second membrane having photosensitizers and a catalyst, f. providing water, g. separating the water from the carbon dioxide in a manner so that hydrogen ions, but not oxygen, may pass from the water to participate in the reduction of the carbon dioxide, and h. illuminating both membranes so as to produce excited photosensitizers to cause electron transfer from the R_(II) /O_(II) solution to the R_(I) /O_(I) solution thence to the carbon dioxide to cooperate with passed hydrogen ions in the reducing of the carbon dioxide to provide at least one product.
 2. A process as described in claim 1 in which the R_(II) /O_(II) redox coupled electrolyte solution is selected from the group of redox couples consisting of H₂ O/O₂, Br⁻ /Br₂, methylene blue, Fe⁺² /Fe⁺³ and EDTA.
 3. A process as described in claim 2 in which the first membrane's photosensitizer is selected from the group of photosensitizers consisting of n-TiO₂, n-Fe₂ O₃, n-WSe₂, p-InP, methylene blue and porphyrins.
 4. A process as described in claim 3 in which the R_(I) /O_(I) redox coupled electrolyte solution is selected from the group of redox couples consisting of I⁻ /I₂, S²⁻ /S_(n) ²⁻, triethanolamine and methyl viologen.
 5. A process as described in claim 4 in which the second membrane's photosensitizer is selected from the group of photosensitizers consisting of p-InP, p-WSe₂, p-GaAs, p-CdTe, n-WSe₂, n-CdSe, Ru(bipy)₃ Cl₂ and phthalocyanines.
 6. A process as described in claim 5 in which the catalyst is on the surface of the second membrane that is in contact with the CO₂ and is selected from the group of catalysts consisting of dichalcogenides of the metals (Ni, Co, Mo, W), SrTiO₃, NADPH, metal coatings (Pb, In), amalgams (Ni, Zn), chrysene, ethyl acrylate, phthalocyanines (Ni, Co), porphyrins and methyl viologen.
 7. A process as described in claim 6 in which the R_(II) /O_(II) couple is Br⁻ /Br₂, the first membrane's photosensitizer is n-WSe₂, the R_(I) /O_(I) couple is I⁻ /I₂ and the photosensitizer in the second membrane is n-WSe₂.
 8. A process as described in claim 6 in which the R_(II) /O_(II) couple is H₂ /H₂ O, the first membrane's photosensitizer is p-InP, the R_(I) /O_(I) couple is S²⁻ /S_(n) ²⁻, and the second membrane's photosensitizer is n-CdSe.
 9. A process as described in claim 6 in which the R_(II) /O_(II) couple is methylene blue, the first membrane's photosensitizer is p-InP, the R_(I) /O_(I) couple is S²⁻ /S_(n) ²⁻, and the second membrane's photosensitizer is n-CdSe.
 10. A process as described in claim 6 in which the R_(II) /O_(II) couple is Fe⁺² /Fe⁺³, the first membrane's photosensitizer is methylene blue, the R_(I) /O_(I) couple is triethanolamine, and the second membrane's photosensitizer is Ru(bipy)₃ Cl₂.
 11. A process as described in claim 6 in which the R_(II) /O_(II) couple is EDTA, the first membrane's photosensitizer is porphyrins, the R_(I) /O_(I) couple is methyl viologen, and the second membrane's photosensitizer is phthalocyanines.
 12. A process as described in claim 6 in which the illuminating step includes exposing both membranes to solar radiation.
 13. A process for reducing carbon dioxide to at least one useful product comprising the steps of:a. providing a R_(II) /O_(II) redox coupled electrolyte solution, b. providing carbon dioxide, c. separating the carbon dioxide from the R_(II) /O_(II) solution with first and second membranes having photosensitizers and the second membrane is in contact with the carbon dioxide and has a catalyst, and d. electrically connecting the first membrane to the second membrane, e. providing water, f. separating the water from the carbon dioxide in a manner so that hydrogen ions, but not oxygen, may pass from the water to participate in the reduction of the carbon dioxide, and g. illuminating both membranes so as to produce excited photosensitizers to cause electron transfer from the R_(II) /O_(II) solution to the carbon dioxide to cooperate with passed hydrogen ions in reducing the carbon dioxide to provide at least one product.
 14. A process as described in claim 13 in which the R_(II) /O_(II) couple is H₂ O/O₂, the first membrane's photosensitizer is n-TiO₂, and the photosensitizer in the second membrane is p-InP.
 15. A process as described in claim 13 in which the R_(II) /O_(II) couple is H₂ O/O₂, the first membrane's photosensitizer is n-TiO₂, and the second membrane's photosensitizer is p-WSe₂.
 16. A process as described in claim 13 in which the R_(II) /O_(II) couple is H₂ O/O₂, the first membrane's photosensitizer is n-Fe₂ O₃, and the second membrane's photosensitizer is p-GaAs.
 17. A process as described in claim 13 in which the R_(II) /O_(II) couple is H₂ O/O₂, the first membrane's photosensitizer is n-Fe₂ O₃, and the second membrane's sensitizer is p-CdTe.
 18. A process as described in claim 13 in which a biasing voltage is provided across the R_(II) /O_(II) electrolyte solution and the water.
 19. A process as described in claim 18 in which the predominant product produced is formic acid.
 20. A process as described in claim 18 in which the predominant product is formaldehyde.
 21. A process as described in claim 18 in which the predominant product is methanol.
 22. A process as described in claim 18 in which the predominant product is methane.
 23. A process as described in claim 13 in which the illuminating step includes exposing the membrane to solar radiation.
 24. Apparatus for reducing carbon dioxide to provide at least one product comprises:means for containing a first redox coupled electrolyte solution, means for containing a second redox coupled electrolyte solution, first means responsive to illumination for transferring electrons from the first electrolyte solution to the second electrolyte solution, means for containing water, means for containing carbon dioxide, means for providing hydrogen ions from the water to the carbon dioxide, and second means responsive to illumination for transferring electrons from the second electrolyte to the carbon dioxide where the transferred electrons and the hydrogen ions cooperate to reduce the carbon dioxide thereby providing at least one product.
 25. Apparatus as described in claim 24 in which the first electron transfer means is a membrane having photosensitizers, and the second electron transfer means is a membrane having photosensitizers and a catalyst.
 26. Apparatus as described in claim 25 in which the first redox coupled electrolyte solution is selected from the group of redox couples consisting of H₂ O/O₂, Br⁻ /Br₂, methylene blue, Fe⁺² /Fe⁺³ and EDTA.
 27. Apparatus as described in claim 26 in which the first membrane's photosensitizer is selected from the group of photosensitizers consisting of n-TiO₂, n-Fe₂ O₃, n-WSe₂, p-InP, methylene blue and porphyrins.
 28. Apparatus as described in claim 27 in which the second redox coupled electrolyte solution is selected from the group of redox couples consisting of I⁻ /I₂, S²⁻ /S_(n) ²⁻, triethanolamine and methyl viologen.
 29. Apparatus as described in claim 28 in which the second membrane's photosensitizer is selected from the group of photosensitizers consisting of p-InP, p-WSe₂, p-GaAs, p-CdTe, n-WSe₂, n-CdSe, Ru(bipy)₃ Cl₂ and phthalocyanines.
 30. Apparatus as described in claim 29 in which the catalyst is on the surface of the second membrane that is in contact with the carbon dioxide and is selected from the group of catalysts consisting of dichalcogenides (Ni, Co, Mo, W), SrTiO₃, NADPH, metal coatings (Pb, In), amalgams (Ni, Zn), chrysene, ethyl acrylate, phthalocyanines (Ni, Co), porphyrins and methyl viologen.
 31. Apparatus as described in claim 30 in which a biasing voltage is provided across the first electrolyte solution and the water in a manner so that the water is anodically biased relative to the first electrolyte solution.
 32. Apparatus as described in claim 31 in which the illumination includes exposing both membranes to solar radiation. 